The present invention relates to optical wiring for information processors such as computers, exchanges or neural networks, and more particularly to an opto-electric integrated circuit which realizes optical wiring which delivers massive optical information at high speed through a free space.
Recently, the processing ability of various information processors including computers and digital exchanges has remarkably been improved, which is mainly achieved by an increase in the density of formed semiconductor integrated circuits and hence a decrease in the propagation distance of signals through the semiconductor integrated circuits, which results in an increase in the processing speed of the semiconductor integrated circuit, based on the progress of semiconductor integration techniques in view of the fact that all signals propagate at a limited speed.
Formation of a large-scaled integrated circuit brought about by maturity of such semiconductor integration techniques increases the number of wiring conductors in a chip or the number of output pins of the chip, which, in turn, increases the number of wiring conductors in the substrate and between the substrate and the devices on the substrate. After all, an increase in the number of interconnected leading conductors in the whole system cannot be avoided. In addition, the problem of cross talk due to mutual interference of signals caused by an increase in the density of formed integrated circuits, and the problems of a clock skew caused by different lengths of the optical wiring conductors caused by an increased operational speed of the device, a delay of propagation of signals, mismatching of the impedances of the signal passageways, and insufficiency of the bands of the signal passageways have become serious.
In order to solve such bottlenecks of the communication, optical wiring techniques are used which have the features including mutual non-inductivity of signals, high resistance to electromagnetic troubles, wide band of interconnected passageway conductors, high speed of signal propagation and elimination of the necessity of grounding. The methods of optical wiring are mainly divided into waveguide type interconnection and free-space type interconnection. The former includes the formation of a waveguide in a two-dimensional plane and optical interconnection of active elements such as semiconductor lasers, photodiodes and/or spatial optical modulation elements (in more detail, see published unexamined Japanese patent applications JP-A-57-15465, JP-A-59-75656, JP-A-60-169167, JP-A-61-156871, JP-A-61-253862 and JP-A-62-181467; "Optical interconnections for massively parallel architectures", Applied Optics, vol. 29, No. 8, pp. 1077-1093 (1990)).
A large-capacity free-space optical interconnection method has been proposed which uses the feature that light is propagatable through a medium-free free space. In the free-space interconnection, solid wiring using a three-dimensional space is possible, so that a great increase in the number of optical wiring conductors is expected compared to electric wiring and waveguide interconnections where the region of the passageway conductors is restricted to within a two-dimensional plane. In this free-space interconnection, two-dimensional active elements (semiconductor laser arrays, photodiode arrays, spatial optical modulation element arrays, drivers) are optically interconnected through passive elements (optical elements having the functions of imaging, and wave separation and combination) (in more detail, see "Optical interconnections for VLSI System" PROCEEDINGS THE IEEE, VOL. 72, NO. 7, JULY (1984), pp. 850-866 (1984); "Surface light emission semiconductor laser" Applied physics", vol. 60, No. 1, pp. 361-367 (1987); "Crossover networks and their optical implementation" Applied optics, vol. 27, No. 15, 1 August 1988, pp. 3155-3160); and Published unexamined Japanese patent applications JP-A-61-212059 and JP-A-61-500941).
The biggest problem of the free space interconnection is alignment. The freedom degree of interconnection in a three-dimensional space gives a three-dimensional freedom degree to alignment, so that an assembly system of bulk elements such as is encountered in a conventional precision optical system is difficult to provide stabilized alignment, and low in productivity. In the free space interconnection, active and passive elements are required to be aligned with high accuracy and then united. To this end, optical integrated elements which provide high alignment are required. The conventional techniques which integrate such optical and electric parts provide stacked planar optics which stack transmission type planar optic components (surface light emitting laser arrays, planar microlens arrays, selfox lenses, photodetection element arrays) having a two-dimensional array structure of FIG. 22 to compose a desired optical circuit, and planar optics which have optical devices formed on a planar surface by an LSI patterning technique (for example, see published unexamined Japanese patent application Japanese Patent Application No. 59-196047; "Stacked planar optics"; "an application of the planar microlens", Applied Optics, vol. 21, No. 19, 10 October (1982), p. 3456); "Integrated optical imagining system", Applied Optics, vol. 29, No. 14, p. 1988 (10 May 1990).
FIGS. 23A and 23B are a conceptual view of a planar optical integrated circuit, which has an optical system which, in turn, has a ceiling-bottom waveguide structure where light from a two-dimensional pattern put on a left-end input surface advances horizontally rightward while repeating total reflection at an upper and a lower inner surface to be imaged by two reflective lenses on a planar surface at a conventional 4-f arrangement incorporated into a thick waveguide. This structure is produced by writing a zone plate-like optical diffraction element, using electron beam exposure. In this case, the positioning of alignment of the respective elements is effected with the accuracy of patterning of the LSI.
FIG. 24 shows a conventional stacked reflective type integrated circuit composed of several superimposed planar substrates to integrate reflective elements compactly (39th Meeting of Applied Physical Society, Spring-lecture preprint No. 30, p-B-9, p. 844).